Canine cardiac myosin was phosphorylated using the myosin light chain kinase from rabbit skeletal muscle myosin. Incorporation of approximately 0.6 moles of phosphate/mole of myosin into the 20,0000 dalton light chain of myosin had very little, if any, effect on the actin-activated ATPase activity. Using the platelet myosin light chain kinase approximately 0.2 moles of phosphate was introduced into the scallop myosin regulatory light chain. BIBLIOGRAPHIC REFERENCES: Maron, B. J., Ferrans, V. J. and Adelstein, R. S.: Isolation and chracterization of myosin from subjects with asymmetric septal hypertrophy. Circulation Research 40: 468, 1977. Adelstein, R. S., Maron, B. J., Daniel, J. L., Conti, M. A., Anderson, W. Jr., and Cohen, E. R. The Phosphorylation of Human Platelet Myosin and Human and Canine Cardiac Muscle Proteins. In Second Joint Symposium on Myocardial Metabolism, Sochi, USSR. 73-84, 1977.